This invention relates to an illuminating system in an exposure apparatus which is used in the fabrication of semiconductor devices to transfer a pattern on a reticle to a substrate having a photoresist layer.
In a conventional illuminating system of an exposure apparatus for the aforementioned purpose, light rays emitted from a light source such as a high-pressure mercury lamp are collected by reflection from an ellipsoidally concave mirror. The reflected and collected rays are passed through a series of lenses including a collimator lens to provide parallel rays which proceed toward a reticle and forms a light beam having a circular cross-sectional shape. This light beam is passed through an optical integrator to uniformalize distribution of light rays over the circular cross-sectional area. The light beam emerged from the integrator serves as a secondary light source which is a plane light source having a circular shape. Then the secondary light source is desirably shaped by an aperture diaphragm, and the shaped beam illuminates the reticle through a condenser lens.
Usually the aforementioned aperture diaphragm has a circular aperture. The degree of coherence of the illuminating light impinging on the reticle is variable with a coherence factor .sigma. which is determined by the diameter of the aperture, and the resolving power and focus depth of the exposure apparatus depend on the coherence factor .sigma.. An optimum value of the degree of coherence is variable according to the shape and size of the pattern on the reticle: for fine patterns, an optimum value of the coherence factor .sigma. ranges from about 0.5 to about 0.7.
With the purpose of improving the resolving power and focus depth of the exposure apparatus, recently studies have been made on the provision of a phase shifter on a transmissive pattern on the reticle to change the phase of the transmitted light. In this case it is favorable to reduce the coherence factor .sigma. to about 0.3 or smaller by narrowing the diameter of the aforementioned aperture for improvements in the resolving power and focus depth.
In the cases of some specific reticle patterns, there is a possibility of improving the resolving power and focus depth of the exposure apparatus by using an annular aperture to shape the secondary light source for illuminating the reticle. The degree of coherence of the illuminating light beam having an annular cross-sectional shape is determined by two coherence factors .sigma. and .sigma.' which are determined by the outer and inner diameters of the beam, respectively, and it has been revealed that an optimum value of .sigma. is about 0.7 while .sigma.' is about 70% of .sigma..
Thus, the best shape and size of the secondary light source (as a plane light source) for illuminating the reticle are variable according to the type of the reticle and the shape and size of the reticle pattern. In the conventional exposure apparatus, the shape and/or size of the secondary light source are varied by using a plurality of aperture diaphragms which are interchangeable and different in aperture shape and/or size. However, irresepective of the aperture shape there is a problem that a considerable portion of the circular light falling on the aperture diaphragm is blocked by the opaque peripheral area of the diaphragm. That is, a considerable loss of illuminating light accompanies the shaping of illuminating light with an aperture diaphragm. Therefore, the illuminance on the reticle becomes relatively low particularly when the aperture diaphragm is relatively large in the opaque area, and consequently the throughput of the exposure apparatus lowers.